Adaptive filters operate on a supplied signal in a prescribe manner such that a desired output signal is generated. Typically, adaptive filters generate a transfer function according to an algorithm that includes updating of the transfer function characteristic in response to an error signal. In this manner, the filter characteristic is optimized to produce a desired result.
When used in an echo canceller, an adaptive filter is used to generate an echo path estimate that is updated in response to an error signal. Echoes commonly occur because of imperfect coupling of incoming signals at the 4-to-2 wire junctions in communications systems. The echoes typically result because the impedance of the 2-wire facility is imperfectly balanced in the 4-to-2 wire junction, causing the incoming signal to be partially reflected over an outgoing path to the source of incoming signals.
Adaptive echo cancellers have been employed to mitigate the echoes by adjusting the transfer function (impulse response) characteristic of an adaptive filter to generate an estimate of the reflective signal or echo and, then, subtracting it from the outgoing signal. The filter impulse response characteristic and, hence, the echo estimate is updated in response to continuously updated samples of the outgoing signal for more closely approximating the echo to be cancelled.
Additionally, double talk detectors (DTD) are generally used to disable adaptation during double talk conditions, that is when both the near end and far end party to a telephone conversation taking place across a telephone line speak simultaneously. Because the double talk detector cannot disable adaptation at the precise instant the double talk occurs, a number of samples occur in the delay period between the commencement of actual double talk and the double talk detector shutting down adaptation. Samples taken during this delay period can and often do perturb the echo path estimate considerably. Also, characteristic changes in the system due to environmental or other causes can also perturb the filtering.
Thus, although prior art arrangements of adaptive filters perform satisfactorily in some applications, often it is impossible to simultaneously achieve both sufficiently fast response and the ability to resist perturbations caused by samples occurring prior to the cessation of adaptation.